                a) Field of the Invention        
The present invention relates to an optical head device which is used for recording on and reproducing from optical recording media. More specifically, it relates to a vibration-proof technology for an objective lens in the optical head device.                b) Description of the Related Art        
As illustrated in FIGS. 5(A) and (B), an optical head device used for the recording and reproducing of optical recording media such as CDs, CD-Rs, and DVDs has a lens holder 3 that holds an objective lens 2, four wires 4 that support the lens holder 3, a drive coil 5 and a drive coil 6 that are fixed to the lens holder 3 for focusing and tracking respectively, and drive magnets 7 and 8 that generate magnetic flux linked to the drive coils 5 and 6. A drive circuit (not illustrated) is arranged on the side of a fixing member (not illustrated), to which the wires 4 are fixed, so that the electricity is supplied to the drive coils 5 and 6 through the wires 4 as the electricity supply lines.
In a popular conventional technology, the objective lens 2 is positioned in the center of the lens holder 3. However, the lens holder 3 in the optical head device illustrated in FIGS. 5(A) and (B) has a coil holding portion 31 that holds the drive coils 5 and 6 and a lens holding portion 35 that projects from the coil holding portion 31 to the front to hold the objective lens 2 on its the top surface.
In the optical head device having such a configuration, once the drive coils 5 and 6 are electrified and a Lorentz force is generated, the lens holder 3 holding the objective lens 2 can be driven in the focusing direction and the tracking direction by a Lorentz force. If the objective lens 2 is vibrated at that time, the recording or reproduction of the optical recording medium cannot be performed accurately. When the objective lens 2 is positioned in the center of the lens holder 3, the center of gravity of the lens holder 3 overlies the center of the objective lens 2. Therefore, sympathetic vibrations seldom occur. Even when sympathetic vibrations are caused, the recording and reproduction of recording media are not affected very much. On the other hand, in the optical head device illustrated in FIGS. 5(A) and (B) having the configuration in which the lens holding portion 35 projects from the coil holding portion 31, sympathetic vibrations are easily caused; since the lens holding portion 35 is lighter than the coil holding portion 31, the objective lens 2 is easily affected by sympathetic vibrations.
Recently used is the technology in which the lens holding portion 35 is made thick to increase rigidity so that the influence of sympathetic vibrations is prevented. However, the current demand for thinner optical head devices brings a demand for a thinner lens holding portion 35, as illustrated in FIGS. 5(A) and (B).
In order to prevent the influence of sympathetic vibrations even when the portion 31 is made thin, the lens holding portion 35 is provided with a wide portion 351 having a width equal to the width of the coil holding portion 31, and a flat surface is so shaped from the wide portion 351 to the front portion 352 that it largely surrounds the objective lens 2 to form a wide area around the objective lens 2. In this manner, the rigidity of the lens holding portion 35 is increased.
Problems Addressed by the Invention
However, when the lens holding portion 35 is made thin and the area of the lens holding portion 35 is made wide, large vibrations resembling vibrations caused on a trampoline become noticeable in the flat surface in the lens holding portion 35.
Also, the present patent applicant has provided a rib 36 to the side portion of the lens holding portion 35, as illustrated in FIGS. 6(A) and (B), to increase rigidity of the root portion of the lens holding portion 35 when the lens holding portion 35 is made thin.
However, if the rib 36 is provided while the lens holding portion 35 is made thin, torsional vibrations are generated on the lens holding portion 35.